STAGES IN PLANT EVOLUTION 47 



pears to be capable of evolution on lines which might 

 have led to the higher plants, is that found in various 

 members of the group of the Protococcoideae among the 

 Algae. The claim of bacteria and other primitive organ- 

 isms of various kinds to the absolute priority of existence 

 is one which is entirely beyond the scope of a book deal- 

 ing with fossil plants. The early evolution of the simple 

 types of the Protococcoidese is also somewhat beyond its 

 scope, but as they appear to lie on the most direct " line 

 of descent " of the majority of the higher plants it cannot 

 be entirely ignored. From the simpler 

 groups of the green Algae other types 

 have specialized and advanced along 

 various directions, but among them 

 there seems an inherent limitation, and 

 none but the protococcoid forms seem 

 to indicate the possibility of really high Fi<r t _ A Proto _ 



development. coccoid Plant consist- 



Ill a few words, a typical example ingfoneceii 

 of one of the simple Protococcoideae P< Protoplasm; , 

 may be described as consisting of a 

 mass of protoplasm in which lie a re- w, ceil wail, 

 cognizable nucleus and a green colouring 

 body or chloroplast, with a cell wall or skin surrounding 

 these vital structures, a cell wall that may at times be 

 dispensed with or unusually thickened according as the 

 need arises. This plant is represented in fig. 17 in a 

 somewhat diagrammatic form. 



In such a case the whole plant consists of one single 

 cell, living surrounded by the water, which supplies it 

 with the necessary food materials, and also protects it 

 from drying up and from immediate contact with any 

 hard or injurious object. When these plants propagate 

 they divide into four parts, each one similar to the 

 original cell, which all remain together within the main 

 cell wall for a short time before they separate. 



If now we imagine that the four cells do not separate, 

 but remain together permanently, we can see the pos- 



